The present invention relates in general to a hammer drill and in particular to a hammer drill of the type which includes a housing and, within the housing, means for guiding a work tool, a driving motor coupled by a gear train and a motion converting wobble disc to a reciprocating driving member acting via a resilient buffer, preferably an air cushion, on an axially reciprocating striking member whose energy is transferred to a work tool inserted in the guiding means.
From U.S. Pat. Nos. 4,280,359; 4,537,264 a hammer drill of this kind is known in which the drum supporting the wobbling disc is supported for rotation on an intermediate shaft which in turn is supported in the housing of the hammer drill. In the gear train between the intermediate shaft and the drum of the wobbling disc there is provided a frictional coupling which is normally disengaged by means of a spring so that the drum with the wobbling disc is not driven via the intermediate shaft: only after pressing the work tool on a processed workpiece a corresponding reaction force becomes effective in counteracting the biasing force of the coupling spring and the coupling is brought in its engaged condition, thus transferring torque from the intermediate shaft to the drum and the wobbling disc and hence to the percussive mechanism of the hammer drill. The moving part of the coupling in this known embodiment consists of a conical shaft end which is fixedly connected to the rotary intermediate shaft. The other coupling part is formed by the drum provided with the wobbling disc, the drum being firmly stayed in axial direction relative to the housing. The effect of the coupling between the driving motor and the driving member of the percussive mechanism depends on pressure exerted by operator against a workpiece. If the pressing force is sufficiently large, the intermediate shaft which acts as a carrier of the movable coupling part is displaced axially against the force of a separation spring into an inner cone of the axially fixed part of the coupling on the drum supporting the wobbling disc and consequently the drum with the wobbling disc and the percussive mechanism are activated. Due to this prior art design the operation of the hammer drill is rendered difficult. Also a reliable engagement of the driving member of the percussive mechanism depends on individually different, nonreproducible circumstances, such as for example different pressing forces applied on the hammer drill by different users. Moreover, due to the support of the drum carrying the wobbling disc, the reaction forces from the percussive mechanism taken up by the housing act in the direction of disengagement of the coupling. Hence, precisely at the point of greatest compression when the percussive mechanism is supposed to apply the largest compressing force and consequently in the range of the drum with wobbling disc to apply the largest torque to the coupling, the reaction force acting in the direction of the disengagement of the coupling has its peak value. For this reason, the operator has to apply relatively large pressing forces on the hammer drill.